The present invention pertains to the signal control art and, more particularly, to a two tone sequential detector which finds particular application in the electrical power management art.
Detectors for determining the presence of control signals are well known in the communication art, and, in the preferred embodiment of the instant invention, find particular application in power management systems. In such utility load management systems, a central controller monitors electrical power being consumed by a plurality of load sites. During periods of peak power consumption it is desirable to disconnect, or "shed" certain deferable loads from the system. This is accomplished by providing at the load sites means to interrupt power to deferable loads, such as water heaters or air conditioners, for a given time interval. A command signal is transmitted by the central controller indicating the need to shed loads. This signal is received by a receiver at the remote site, detected, thereafter activating a timer which "times out" the load for a given time interval.
The command signal according to the preferred embodiment of the invention is of the two tone sequential type, i.e. a tone of a first frequency followed immediately by a tone of a second frequency. The detector at the load site must identify the two tone sequence in order to initiate the load shed interval.
Two tone sequential detectors are well known, particularly in the communication signalling art. Such prior art detectors indicate the presence of the two tone command signal if both of the following criteria are satisfied. First, the initial tone must be present for some predetermined time interval. Secondly, the second tone must be present for a given time interval following conclusion of the first tone. A fundamental problem with the prior art two tone detectors has been their susceptibility to falsing. Given the above criteria, it is evident that falsing might occur, for example, under various noise conditions. Thus, there is a need in the two tone signalling art to provide a high security detector.
In addition, in some applications it is desirable to control one of two functions based upon the order in which the two tones are set. Thus, here it is desirable to provide a high security two tone detector which conveniently and inexpensively can detect the sequence of incoming tones to thereby control appropriate output functions.